Anhydrous ammonia is the nitrogen source for most of the synthetic fertilizers and is widely used in its natural state for pre-plant nitrogen on some of the larger grain crops--corn, milo, sorghum, etc. A farmer in the corn belt has two choices for application; one in the fall after the weather has turned cold. The other just as soon as the soil thaws in the spring.
Anhydrous ammonia stored in a pressure vessel is in the liquified gas phase, a liquid due to its own vapor pressure. Any heat flow into the system will be absorbed in the system by additional formation of vapor, increasing the system pressure. Heat flow out of the system will produce condensation of the vapor reducing the system pressure.
The application and distribution of ammonia for fertilizer is usually accomplished by a tractor equipped with a tool bar or cultivator with means to meter and place the fertilizer in rows 4" to 8" beneath the soil surface, means to seal the soil and an ammonia wagon in tow with suitable flexible connections to transfer the ammonia from the ammonia tank through the meter on the tool bar.
The energy required to move the ammonia through the system is supplied by the vapor pressure in the tank; however, the ammonia moving from the tank to the meter experiences a pressure drop equal to the pressure required to accelerate the liquid to its velocity through the system. This pressure drop requires a temperature drop in the system which is provided by refrigeration or vaporization within the system. The colder the day, the lower the pressure, the greater the percentage of vapor in the system prior to the meter.
Operators using the larger more powerful tractors available today, capable of pulling wider tool bars at greater speeds across the fields, have outrun the capacities of available ammonia placement system. Much of the late fall and early spring ammonia application is performed with ambient temperatures between 35.degree. and 45.degree. and vapor pressure of 5 atmospheres (approximately 60 PSIG). These are very undesirable metering conditions, and in too many instances, the farmer finds he must reduce his speed or make two passes to apply the desired quantity of fertilizer. Neither procedure is very economical.
While most of the metering devices in use today are capable of handling enough fertilizer with ambient temperatures around 75.degree. and vapor pressure of 9 atmospheres (approximately 120 PSIG). With their internal resistance, abrupt turns, accelerating and decelerating the ammonia, they become little more than a portion of a conduit which transfers the fertilizer from the tank to the fields when used with the lower system pressures.
The early art used a tractor-mounted, self-contained applicator. The meter mounted on the withdrawal valve of the tank established a known fixed resistance from the liquid source to the meter. The use of temperature gradient meter dials were fairly successful with the speeds and swath widths required at that time.
As the tool bar concept replaced the applicators, moving the meter from the tank to the tool bar, the known resistance prior to metering became the unknown.
Under ideal soil conditions, the rate of application may be in excess of 30 gallons per minute or a 1,000 gallon wagon every 35 minutes. Meter setting charts are attached to the tool bar and with the swatch width fixed, the charts show tractor speed against pounds N per acre. With vapor pressures of 80 PSIG and up, the metering is fairly constant, but 60 PSIG and under, there is considerable variation and experienced operators under certain ambient conditions will change the meter setting two or three times with each tank. Therefore, a simple dial with the two pressure bases is a helpful tool, providing good control at the higher temperatures as well as a usable reference at the lower temperatures.